1. Field of the Invention
The present invention relates to a method for predicting a lifetime of a filament of anion source used in an ion implanter, and an ion source device having a device for predicting the lifetime of the filament.
2. Description of the Related Art
Hitherto, a lifetime of a filament for emitting thermoelectrons, which forms an ion source used by an ion implanter or the like is predicted in a manner that current (filament current) flowing through the filament is constantly measured during the operation of the ion source, the measured current values are each compared with a predetermined reference value, and a time left till the filament will be broken is estimated. The predetermined reference value is the past data on the filament current of the ion source using a filament having the same characteristics as those of the filament under measurement. A technique similar to the above one is described also in JP-A-04-306544 (paragraph Nos. [0017] and [0022], FIG. 1) given below.
The reason why the above-mentioned method has been used follows. To operate the ion source at a fixed output level (The output level corresponds to an ion beam current derived from the ion source in this specification), it is necessary to set the amount of thermoelectrons emitted from the filament at a fixed value. The amount of thermoelectrons is observed in the form of an arc current flowing between the filament and a plasma production chamber. To keep the arc current constant, in this type of ion source, usually, the amount of electric power fed to the filament is controlled to be constant by controlling the filament current.
In this case, as the operation of the ion source progresses, the filament becomes thin by the sputtering by ions in the plasma generated in the plasma production chamber and its evaporation. For example, the filament is gradually reduced in diameter at the center portion thereof. As a result, resistance of the overall filament increases, so that the filament current required for keeping the electric power fed to the filament at the fixed value, becomes smaller.
Accordingly, the related-art technique assumes that as the filament current becomes small, the diameter of the filament becomes small in diameter and hence, breaking of the filament more easily occurs. The filament current of the filament of the ion source under measurement is compared with a filament current at which the filament was broken in the reference ion source using the filament having the same characteristics as those of the filament under measurement. In this way, a time left till the filament will be broken is predicted.
The above-mentioned technical approach is correct, provided that the output level of the ion source is always constant. However, it is a rare case that an actual ion source used for the ion implanter or the like is operated in such a state, in other words, at the fixed output level.
In this connection, in the case of, for example, an ion source which is used for an ion implanter, called a medium-current ion implanter, an output of the ion source, i.e., an ion beam current derived from it, relatively widely varies from a maximum output level of 20 mA or higher to a minimum output level of about 0.1 mA.
The output of the ion source depends on the amount of thermoelectrons emitted from the filament. The filament current must be varied in accordance with an output required for the ion source. Accordingly, where the diameter of the filament is equal, when the output of the ion source is large, the filament current is large, and when it is small, the filament current is small. In other words, to decrease the output of the ion source, i.e., the ion beam current, it is necessary to decrease the arc current. To this end, it is necessary to reduce the filament current. In this case, its appears as if the lifetime of the filament is rapidly reduced in the case where the lifetime of the filament is predicted based on the measured filament current as in the related-art method. This is not correct because the filament current is merely reduced.
Thus, in the case where the output level of the ion source is not constant, the related-art technique for predicting the lifetime of the filament by measuring the filament current cannot predict the lifetime of the filament exactly.